Viola riviniana is known in plant parlance as the Common Dog Violet. Not a very distinguished name for a lovely little flower- perhaps it wasn't held in such high regard as the Sweet Violet (Viola ororata)? Dog Violets have no scent whereas Sweet Violets are indeed sweetly scented.
 V. riviniana is one of those country wildflowers that does fine in towns and cities too. The patch above was one of several I saw in some neglected front gardens near the Old Kent Road. Needless to say certain gardening books inform you that Dog Violets are weeds that need to be eradicated from your lawn but why would you?